Solder glass and electrical device employing same

ABSTRACT

A solder glass comprising, by weight, about 65% Sb 2  O 3  ; about 30% B 2  O 3  ; and about 5% PbO. The glass is molten at about 350° C. and can be employed as a molten seal in electrical devices such as tungsten-halogen lamps.

TECHNICAL FIELD

This invention relates to solder glasses and more particularly to suchglasses which become molten at about 350° C. and can be employed withquartz-to-metal seals in electrical devices.

BACKGROUND ART

Certain electrical devices, e.g., tungsten halogen lamps, employenvelopes comprising a high percentage of silica, such as fused silica,fused quartz or Vycor, the latter being a 96% silica glass. To achieve ahermetic seal between the glass and the lead-in conductors of theelectrical device it is common practice to employ a pinch seal. Thelead-in conductors generally comprise a very thin foil portion ofmolybdenum which forms the actual hermetic seal and an outer lead-wireportion of a refractory metal which can also be molybdenum. One end ofthe lead-wire is attached to the foil and the other end extendsexteriorly of the pinch. This relatively heavy outer lead-wire does notform a hermetic seal with the quartz because of its relatively largediameter and the difference in thermal expansion coefficients betweenthe two. In actual practice, because the formation of the pinch sealdoes not allow the quartz to flow completely around and against the fullperiphery of the outer lead-wire, a small capillary passage is lefttherebetween. The thin foil is thus exposed to atmospheric oxygenentering via the capillary passages. At elevated temperatures, say aboveabout 350° C., oxidation of the foil can occur, resulting in breakage ofthe electrical connection to the outer lead-wire.

It has been suggested that this problem can be reduced by filling thecapillary with a solder glass which becomes molten when the device isoperated, thus forming a molten seal. Specifically, a lead borate glasshas been suggested. Lead borate glasses have lead contents above 70%;however, the use of such a glass requires the use of platinum orplatinum clad lead-wires since lead borate attacks molybdenum. Further,U.S. Pat. No. 3,588,315 suggests binary glasses such as antimony borateand ternary glass compositions of antimony borate with the addition ofsmall amounts of molybdenum trioxide or tungsten trioxide.

It would be an advance in the art if additional solder glasses could bemade available.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to provide a solder glasshaving a melting temperature at about 350° C. and which is notdeleterious to molybdenum.

These objects are accomplished, in one aspect of the invention, by theprovision of a solder glass comprised of Sb₂ O₃, B₂ O₃ and PbO. Thisglass has the requisite melting temperature and is not deleterious tomolybdenum. Thus, as a seal for electrical devices, it easily fills thecapillary passages.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a partial, sectional view, somewhat exaggerated asa size, of the end of an electrical device employing the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawing.

Referring now to the drawing with greater particularity there is shown aseal for an electrical device 10, which can be a tungsten-halogen lamp.Only a single end of the device 10 is illustrated, it being understoodthat in a double-ended device the end not shown would be similar. It isalso to be understood that the invention is equally applicable tosingle-ended devices wherein a plurality of lead-in connectors areadjacent to one another. Device 10 has a body 11 of substantially fusedsilica or quartz or some other high silica content glass. By high silicacontent glass is meant a glass having more than 90% silica.

At least one electrically conductive member 12 is sealed in the end ofdevice 10. The member 12 comprises a proximal portion 14 which projectswithin the body 11, an intermediate thin foil portion 16, as ofmolybdenum which forms the hermetic seal with body 11, and a distalportion 18 which projects exteriorly of the body 11.

As noted above, the formation of such a pinch seal leaves a smallcapillary passage 20 surrounding the distal portion 18. The passage 20may have a width on the order of 0.001 inch.

To prevent oxidation of the foil 16 at elevated temperatures, i.e.,above 350° C., the capillary passage 20 is filled with a solder glass22. The glass 22 comprises about 65 weight percent Sb₂ O₃ ; about 30weight percent B₂ O₃ ; and about 5 weight percent PbO. This solder glassbecomes molten at about 350° C. and thus protects the foil 16; further,the small quantity of lead does not effect the molybdenum seal.

The glass is formulated by weighing out the antimony trioxide (Sb₂ O₃)and the lead oxide (PbO). The boron trioxide (B₂ O₃) is supplied fromboric acid (H₃ BO₃) at the rate of 1.776 grams of the acid for each gramof oxide desired. The materials are thoroughly mixed together and thenmelted. The melting is preferably carried out in a suitable crucible,such as one of kyanite (Al₂ O₃ -SiO₂), pure alumina or platinum with 10%rhodium. The melts can be made by introducing the batch, in smallportions, into a crucible heated to about 850° C. Additional batch isadded after the initial melting reaction begins until the crucible isfull. The melts were made in air. The melt is held at the meltingtemperature of 850° C. for about fifteen minutes after it appears batchfree and is then stirred with a fused quartz rod. The temperature isthen lowered about 50° C. and held for about fifteen minutes and themelt is then cast into thin rods or washers. Alternatively, theviscosity of the melt could be controlled and rods drawn directly fromthe crucible.

The solder glass so made can be applied to the seal by positioning thedevice 10 vertically with the capillary to be filled uppermost. The body11, at the seal area, is heated and the solder glass is applied theretoand caused to run down into the capillaries. If the device isdouble-ended, it is then inverted and the procedure repeated.

While there have been shown what are at present considered to bepreferred embodiments of the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madeherein without departing from the scope of the invention as defined bythe appended claims.

We claim:
 1. A solder glass comprising, by weight; about 65% Sb₂ O₃ ;about 30% B₂ O₃ ; and about 5% pbO.
 2. A seal for electrical devicescomprising a body of substantially fused silica and at least oneelectrically conductive member sealed therein, said member comprising aproximal portion projecting within said body, an intermediate thin foilportion hermetically sealed within said body, and a distal portionprojecting exteriorly of said body, and a capillary passage surroundingsaid distal portion and extending to said intermediate portion, saidcapillary passage being substantially filled with a solder glasscomprising: about 65 weight percent Sb₂ O₃ ; about 30 weight percent B₂O₃ ; and about 5 weight percent PbO.